In recent years, glass substrates are used in various industrial fields besides conventional applications. In the field of various electronic devices, for example, glass substrates suitable for respective applications are used.
For example, personal computers (PC) or the like are provided with a hard disk drive (HDD) or the like as an external storage device. In general, the hard disk drive has mounted thereon a magnetic disk which is known as a storage for computer or the like. The magnetic disk has a configuration in which a magnetic layer or the like is formed on an appropriate substrate, for example, an aluminum based alloy substrate.
In recent years, glass substrates which are materials having high strength and high rigidity have been frequently used in place of rigid brittle metal substrates. Further, the glass substrates have attracted attention as substrates for magnetic disks for a server application.
Furthermore, with a progress of growing memory capacity of hard disks, in order to increase memory density, to improve record reading accuracy, and the like, higher-accuracy flatness has been demanded for the glass substrates for magnetic disks, and particularly, it has been demanded that micro-waviness largely affecting floating performance of magnetic heads is more decreased.
As a polishing machine for producing such glass substrates, patent document 1 discloses a double-sided polishing machine as shown in FIG. 10. This double-sided polishing machine 100 has a polishing carrier mounting unit having an internal gear 101 and a sun gear 102 which are each driven for rotation at a predetermined rotation ratio, and an upper surface plate 103 and a lower surface plate 104 which are driven for rotation reversely to each other with this polishing carrier mounting unit therebetween. Polishing pads 106 are each mounted on faces of the upper surface plate 103 and the lower surface plate 104 facing to glass substrates 105. Polishing carriers 107 mounted so as to be meshing engagement with the internal gear 101 and the sun gear 102 perform a planetary gear movement. In this planetary gear movement, the polishing carrier 107 moves while rotating on a center of itself as its axis and moving around the sun gear 102 as an axis. Both faces of the glass substrate 105 are concurrently polished with friction between the polishing pads 106 and the glass substrate 105 caused by this planetary gear movement. In this double-sided polishing machine 100, it is necessary to move up the upper surface plate 103, in order to take out the polished glass substrate 105 after the glass substrate 105 have been polished.
In polishing machine such as that described above, glass substrates are polished while supplying a predetermined abrasive to the polishing pads. As such abrasives for glass substrates, for example, ceria particle (cerium oxide particle) abrasives and silica particle (silicon dioxide particle) abrasives are known.
Glass substrates for use in electronic devices and the like, such as the glass substrates for magnetic disks described above, are required to have high precision and high quality, and there is severe competition in development. Patent documents 1 to 3 disclose a process for producing such a glass substrate or a method of polishing for producing the substrate.    Patent Document 1: JP-A-2008-103061    Patent Document 2: JP-A-2006-265018    Patent Document 3: JP-A-2000-163740